单项选择题 A heavy，thick-walled tube，usually steel，used between the drill pipe and the bit in the drill stem to weight the bit in order to improve its performance is called a（）.

单项选择题 A heavy steel curved arch constructed athwartships and above the after deck on a towing vessel is sometimes called a（）.

单项选择题 A head-on situation at night is one in which you see dead ahead a vessel showing（）.